System Freezes - Hard Reset Required

Hello everyone,
I have a dilemma with my system. It randomly freezes while I'm using it and requires a hard reset. It does not blue screen, it does not get slow and it does NOT unfreeze no matter how long you allow it to sit. It seems like it freezes more often when I'm running heavy graphics, but it could just be that the load is higher overall.

I have run memtest for days on end with no errors, updated the BIOS, adjusted the voltage settings (a recommendation I found for the motherboard and memory) and tested under multiple operating systems (Windows XP 64-bit, Windows Vista Ultimate 64-bit, Knoppix, Ubuntu) and received the same results.

Here are all the components, currently running on Windows XP Professional 64-bit:

Motherboard: XFX nForce 790i Ultra SLI
Memory: 4x 2GB Patriot 1333MHz DDR3 7-7-7-20
Video: 2x EVGA nVidia GeForce 9600GT 1GB DDR3
CPU: Intel Q9300 2.5 GHz
PSU: Enermax 1000W
HD: 3x 500 GB Western Digital (RAID5)

I've also thrown in extra fans, a Zalman CPU fan/heatsink and a couple DVD drives. The CPU rarely gets above 50 C.

At this point I'm thinking it's either a bad motherboard, two bad video cards or possibly some kind of conflict between the motherboard's RAID and nVidia's software storage controller (which I've disabled, but still shows up). I may be testing the RAID soon (by reinstalling to a single drive) but I'm not sure how to test the other hardware.

I don't know how easily this can be solved, but I would greatly appreciate any support and I'll do my best to answer any questions you have. Thank you!

Got a few blue screens when the video drivers updated but they went away with a fresh install, so I doubt that was relevant. Other than that, no error messages, no blue screens.

I'll check the GPU temp after the next crash (very soon)

Edit: Temperatures after crash:
CPU: 48C
GPU: 47C

Edit2: I have also tried disabling SLI and running with one graphics card (first tried one, then the other). None of that seemed to make a difference. I'm leaning towards a bad motherboard at this point, but I don't feel confident I did everything I could to change voltages and other settings. Also, I'm not sure how to run an overall diagnostic against a mobo (or if you even can).

manually set everything in the bios - "auto" settings are utterly useless, also give your ram that extra 0.1v of power and set its ratio to 1:1

also things to try - another PSU, and test with prime with a single stick of ram (try 1 stick, then the other separately etc)

A couple of things come to mind to me. First, these components scream heat. Try running with the side off the case for a while.

The second, and most important, is where you said you turned speed stepping off. Turn it back on so your processor throttles like it should.

The north branch heat sink might not be doing a good job. Lots of work through that area causing lots of heat.

When you say "these are temps after crash".... so what. What are they just before crash. You lose a lot of heat during crash and reboot.

EDIT: I had some of these problems before I lost a processor.......
EDIT II: a while back there were some problems with nvidia mother board drivers and raid set ups. Can't remember any more what it was.

Ack! New error message in BIOS:

I have not had time to reinstall yet. So really nothing that I'm sure of has changed....